A team of nanoengineers at the University of California San Diego recently developed a safety feature that keeps lithium metal batteries from overheating and catching fire in the case of an internal short circuit.
A small tweak was made to a distinct battery part known as the separator. The component works as a barrier between the cathode and anode to slow down the energy flow that rises inside the battery during short circuits. The team was led by a nano-engineering professor, Ping Liu, from UC San Diego along with his PhD student Matthew Gonzalez.
“We’re not trying to stop battery failure from happening. We’re making it much safer so that when it does fail, the battery doesn’t catastrophically catch on fire or explode,” said Gonzalez.
Repeated charging leads to the growth of needle-like structures (dendrites) on the anode. It is one of the main reasons behind the failure of lithium metal batteries. As time goes by, the dendrites grow so long that they puncture the separator creating a bridge between the cathode and anode. This causes an internal short circuit.
Internal short circuits result in an excessive flow of electrons between two electrodes, causing the battery to instantly overheat and stop working. To resolve this issue, the UC San Diego team developed a separator that softens this occurrence. One side of this separator is covered by a very thin, partially conductive network of carbon nanotube, which intercepts the formation of any dendrites in the anode.
Whenever any dendrite grows to pierce through the separator and this web, the nanotubes provide a freeway to excessive electrons that slowly drain out through them rather than rushing straight into the cathode.
Gonzalez compared this new battery separator with a spillway at a dam. “When a dam starts to fall, a spillway is opened up to let some of the water trickles out in a controlled fashion so that when the dam does break and spill out, there’s not a lot of water left to cause a flood.”
He added: “That’s the idea with our separator. We are draining out the charge much, much slower and prevent a ‘flood’ of electrons to the cathode. When a dendrite gets intercepted by the separator’s conductive layer, the battery can begin to self-discharge so that when the battery does short, there’s not enough energy left to be dangerous.”
Other efforts have focused on the creation of separators that block dendrites from piercing through them. However, these separators still need pores to let ions flow through them, otherwise the battery won’t work. This prolongs the inevitable but does nothing to evade it. Eventually, when the dendrites pierce through the wall, the resulting short circuit is worse than normal.
The UC San Diego team took a more neutral approach to this issue. Rather than blocking the dendrites, they tried to mitigate their effects and slow down the rate of battery failure.
The lithium metal batteries equipped with the new separator developed by this team showed signs of gradual failure over 20 o 30 cycles. Meanwhile, batteries carrying normal separator failed abruptly in a single cycle.
“In a real use case scenario, you wouldn’t have any advance warning that the battery is going to fail. It could be fine one second, then catch on fire or short out completely the next,” said Gonzales. “It’s unpredictable. But with our separator, you would get advance warning that the battery is getting a little bit worse, a little bit worse, a little bit worse, each time you charge it.”